There is increasing evidence that opening the mitochondrial ATP-sensitive K+ channel (mitoKATP) in heart is cardioprotective in ischemia-reperfusion injury. The long-term goals of this proposal are to uncover the mechanisms by which mitoKATP exerts its cardioprotective effects. Specific aims are: To test and extend the hypothesis that mitoKATP is the site of cardioprotection. To test the hypothesis that the effects of mitoKATP opening/closing on cardiomyocytes are due to small changes in mitochondrial K+ flux. To determine how mitoKATP opening prior to ischemia acts as a "trigger" of cardioprotection and how mitoKATP opening increases generation of reactive oxygen species. To determine whether the endogenous signaling pathways that open mitoKATP act by phosphorylating the channel. To determine the role and mechanisms of mitoKATP as an end effector of cardioprotection. The unifying principle behind these aims is that the consequences of mitoKATP opening depend strongly on the underlying bioenergetic state, in particular, on whether mitochondrial membrane potential (delta psi) is high or low when mitoKATP is opened. This accounts for the finding that mitoKATP opening plays two distinct roles in cardioprotection as both a trigger and an end effector of preconditioning. In the trigger phase, mitoKATP opening causes increased generation of reactive oxygen species. During ischemia and reperfusion, mitoKATP opening regulates energy transfers from mitochondria to the cytosol. The experimental approach is to study the problem from the ground up - from measurements of K+ flux through the purified protein through bioenergetic studies on mitochondria and permeabilized fibers to physiological studies on the cardiomyocyte and perfused heart.